2.1. The Complement System
The complement system is a group of proteins that constitutes about 10 percent of the globulins in the normal serum of humans (Hood, L. E., et al., 1984, Immunology, 2d Ed., The Benjamin/Cummings Publishing Co., Menlo Park, Calif., p. 339). Complement (C) plays an important role in the mediation of immune and allergic reactions (Rapp, H. J. and Borsos, T, 1970, Molecular Basis of Complement Action, Appleton-Century-Crofts (Meredith), N.Y.). The activation of complement components leads to the generation of a group of factors, including chemotactic peptides that mediate the inflammation associated with complement-dependent diseases. The sequential activation of the complement cascade may occur via the classical pathway involving antigen-antibody complexes, or by an alternative pathway which involves the recognition of certain cell wall polysaccharides. The activities mediated by activated complement proteins include lysis of target cells, chemotaxis, opsonization, stimulation of vascular and other smooth muscle cells, and functional aberrations such as degranulation of mast cells, increased permeability of small blood vessels, directed migration of leukocytes, and activation of B lymphocytes and macrophages (Eisen, H. N., 1974, Immunology, Harper & Row Publishers, Inc. Hagerstown, Md., p. 512).
During proteolytic cascade steps, biologically active peptide fragments, the anaphylatoxins C3a, C4a, and C5a (See WHO Scientific Group, 1977, WHO Tech. Rep. Ser. 606:5 and references cited therein), are released from the third (C3), fourth (C4), and fifth (C5) native complement components (Hugli, T. E., 1981, CRC Crit. Rev. Immunol. 1:321; Bult, H. and Herman, A. G., 1983, Agents Actions 13:405).
2.2. The C3b/C4b Complement Receptor (CR1)
The human C3b/C4b receptor, termed CR1, is present on erythrocytes, monocytes/macrophages, granulocytes, B cells, some T cells, splenic follicular dendritic cells, and glomerular podocytes (Fearon, D. T., 1980, J. Exp. Med. 152:20, Wilson, J. G., et al., 1983, J. Immunol. 131:684; Reynes, M., et al., 1985, J. Immunol. 135:2687; Gelfand, M. C., et al., 1976, N. Engl. J. Med. 295:10; Kazatchkine, M. D., et al., 1982, Clin. Immunol. Immunopathol. 27:170). CR1 specifically binds C3b, C4b, and iC3b. A soluble form of the receptor has been found in plasma that has ligand binding activity and the same molecular weight as membrane-associated CR1 (Yoon, S. H. and Fearon, D. T., 1985, J. Immunol. 134:3332). CR1 binds C3b and C4b that have covalently attached to immune complexes and other complement activators, and the consequences of these interactions depend upon the cell type bearing the receptor (Fearon, D. T. and Wong, W. W., 1983, Ann. Rev. Immunol. 1:243). Erythrocyte CR1 binds immune complexes for transport to the liver (Cornacoff, J. B., et al., 1983, J. Clin. Invest. 71:236; Medof, M. E., et al., 1982, J. Exp. Med. 156:1739) CR1 on neutrophils and monocytes internalizes bound complexes, either by adsorptive endocytosis through coated pits (Fearon, D. T., et al., 1981, J. Exp. Med. 153:1615; Abrahamson, D. R. and Fearon, D. T., 1983, Lab. Invest. 48:162) or by phagocytosis after activation of the receptor by phorbol esters, chemotactic peptides, or proteins that are present in the extracellular matrix, such as fibronectin and laminin (Newman, S. L., et al., 1980, J. Immunol. 125:2236; Wright, S. D. and Silverstein, S. C., 1982, J. Exp. Med. 156:1149; Wright, S. D., et al., 1983, J. Exp. Med. 158:1338). Phosphorylation of CR1 may have a role in the acquisition of phagocytic activity (Changelian, P. S. and Fearon, D. T., 1986, J. Exp. Med. 163:101). The function of CR1 on B lymphocytes is less defined, although treatment of these cells with antibody to CR1 enhanced their response to suboptimal doses of pokeweed mitogen (Daha, M. R., et al., 1983, Immunobiol. 164:227 (Abstr.)). CR1 on follicular dendritic cells may subserve an antigen presentation role (Klaus, G. G. B., et al., 1980, Immunol. Rev. 53:3).
CR1 can also inhibit the classical and alternative pathway C3/C5 convertases and act as a cofactor for the cleavage of C3b and C4b by factor I, indicating that CR1 also has complement regulatory functions in addition to serving as a receptor (Fearon, D. T., 1979, Proc. Natl. Acad. Sci. U.S.A. 76:5867; Iida, K. and Nussenzweig, V., 1981, J. Exp. Med. 153:1138). In the alternative pathway of complement activation, the bimolecular complex C3b,Bb is a C3 activating enzyme (convertase). CR1 (and factor H, at higher concentrations) can bind to C3b and can also promote the dissociation of C3b,Bb. Furthermore, formation of C3b,CR1 (and C3b,H) renders C3b susceptible to irreversible proteolytic inactivation by factor I, resulting in the formation of inactivated C3b (iC3b). In the classical pathway of complement activation, the complex C4b,2a is the C3 convertase. CR1 (and C4 binding protein, C4bp, at higher concentrations) can bind to C4b, and can also promote the dissociation of C4b,2a. The binding renders C4b susceptible to irreversible proteolytic inactivation by factor I through cleavage to C4c and C4d (inactivated complement proteins.)
CR1 is a glycoprotein composed of a single polypeptide chain. Four allotypic forms of CR1 have been found, differing by increments of .about.40,000-50,000 daltons molecular weight. The two most common forms, the F and S allotypes, also termed the A and B allotypes, have molecular weights of 250,000 and 290,000 daltons (Dykman, T. R., et al., 1983, Proc. Natl. Acad. Sci. U.S.A. 80:1698; Wong, W. W., et al., 1983, J. Clin. Invest. 72:685), respectively, and two rarer forms have molecular weights of 210,000 and &gt;290,000 daltons (Dykman, T. R., et al., 1984, J. Exp. Med. 159:691; Dykman, T. R., et al., 1985, J. Immunol. 134:1787). These differences apparently represent variations in the polypeptide chain of CR1, rather than glycosylation state, because they were not abolished by treatment of purified receptor protein with endoglycosidase F (Wong, W. W., et al., 1983, J. Clin. Invest. 72:685), and they were observed when receptor allotypes were biosynthesized in the presence of tunicamycin (Lublin, D. M., et al., 1986, J. Biol. Chem. 261:5736). All four CR1 allotypes have C3b-binding activity (Dykman, T. R., et al., 1983, Proc. Natl. Acad. Sci. U.S.A. 80:1698; Wong, W. W., et al., 1983, J. Clin. Invest. 72:685; Dykman, T. R., et al., 1984, J. Exp. Med. 159:691; Dykman T. R., et al., 1985, J. Immunol. 134:1787).
Two nonoverlapping restriction fragments of a CR1 cDNA were shown to crosshybridize under conditions of high stringency (Wong, W. W., et al., 1985, Proc. Natl. Acad. Sci. U.S.A. 82:7711). Both cDNA probes also hybridized to multiple restriction fragments of genomic DNA, most of which were common to both probes (id.). The existence of repetitive coding sequences within CR1 was confirmed by sequence comparisons (Klickstein, L. B., et al., 1985, Complement 2:44 (Abstr.)). In addition, the CR1 gene has been shown to have repetitive intervening sequences by the demonstration of crosshybridization of a genomic probe lacking coding sequences to several genomic restriction fragments (Wong, W. W., et al., 1986, J. Exp. Med. 164:1531). Further, DNA from an individual having the larger S allotype had an additional restriction fragment hybridizing to this genomic probe when compared with DNA from an individual having the F allotype, suggesting that duplication of genomic sequences occurred in association with the higher molecular weight CR1 allele (id.).
CR1 has been shown to have homology to complement receptor type 2 (CR2) (Weis, J. J., et al., 1986, Proc. Natl. Acad. Sci. U.S.A. 83:5639-5643).
2.3. Abnormalities of CR1 in Human Disease
Diminished expression of CR1 on erythrocytes of patients with systemic lupus erythematosus (SLE) has been reported by investigators from several geographic regions, including Japan (Miyakawa et al., 1981, Lancet 2:493-497; Minota et al., 1984, Arthr. Rheum. 27:1329-1335), the United States (Iida et al., 1982, J. Exp. Med. 155:1427-1438; Wilson et al., 1982, N. Engl. J. Med. 307:981-986) and Europe (Walport et al., 1985, Clin. Exp. Immunol. 59:547; Jouvin et al., 1986, Complement 3:88-96; Holme et al., 1986, Clin. Exp. Immunol. 63:41-48). Taken as a group, patients have an average number of receptors per cell that is 50-60% that of normal populations. An early report noted that CR1 number on erythrocytes varied inversely with disease activity, with lowest numbers occurring during periods of most severe manifestations of SLE, and higher numbers being observed during periods of remission in the same patient (Iida et al., 1982, J. Exp. Med. 155:1427-1438). CR1 number has also been found to correlate inversely with serum levels of immune complexes, with serum levels of C3d, and with the amounts of erythrocyte-bound C3dg, perhaps reflecting uptake of complement-activating immune complexes and deposition on the erythrocyte as an "innocent bystander" (Ross et al., 1985, J. Immunol. 135:2005-2014; Holme et al., 1986, Clin. Exp. Immunol. 63:41-48; Walport et al., 1985, Clin. Exp. Immunol. 59:547). A patient with SLE lacking CR1 on erythrocytes was found to have an auto-antibody to CR1 (Wilson et al., 1985, J. Clin. Invest. 76:182-190). Decreased titers of the anti-CR1 antibody coincided with improvement of the patient's clinical condition and with partial reversal of the receptor abnormality. Anti-CR1 antibody has been detected in two other SLE patients (Cook et al., 1986, Clin. Immunol. Immunopathol. 38:135-138). Recently, acquired loss of erythrocyte CR1 in the setting of active SLE and hemolytic anemia was demonstrated by observing the rapid loss of the receptor from transfused erythrocytes (Walport et al., 1987, Clin. Exp. Immunol. 69:501-507 ).
The relative loss of CR1 from erythrocytes has also been observed in patients with Human Immunodeficiency Virus (HIV) infections (Tausk, F. A., et al., 1986, J. Clin. Invest. 78:977-982) and with lepromatus leprosy (Tausk, F. A., et al., 1985, J. Invest. Dermat. 85:58s-61s).
Abnormalities of complement receptor expression in SLE are not limited to erythrocyte CR1. Relative deficiencies of total cellular CR1 of neutrophils and plasma membrane CR1 of B lymphocytes of the SLE patients have been shown to occur (Wilson et al., 1986, Arthr. Rheum. 29:739-747).
In patients with Type IV SLE nephritis, all detectable CR1 antigen is lost from podocytes, whereas in less severe forms of SLE nephritis and in non-SLE types of proliferative nephritis, including membranoproliferative glomerulonephritis Types I and II, CR1 expression on glomerular podocytes does not differ from normal (Kazatchkine et al., 1982, J. Clin. Invest. 69:900-912; Emancipator et al., 1983, Clin. Immunol. Immunopathol. 27:170-175). However, patients having Type IV SLE nephritis do not have fewer numbers of erythrocyte CR1 than do SLE patients having other types of renal lupus or no nephritis (Jouvin et al., 1986, Complement 3:88-96).
In vivo complement activation up-regulates CR1 expression at the plasma membrane of neutrophils (Lee, J., et al., 1984, Clin. Exp. Immunol. 56:205-214; Moore, F. D., Jr., et al., 1986, N. Engl. J. Med. 314:948-953).
Complement activation has also been associated with disease states involving inflammation. The intestinal inflammation of Crohn's disease is characterized by the lymphoid infiltration of mononuclear and polymorphonuclear leukocytes. It was found recently (Ahrenstedt et al., 1990, New Engl. J. Med. 322:1345-9) that the complement C4 concentration in the jejunal fluid of Crohn's disease patients increased compared to normal controls. Other disease states implicating the complement system in inflammation include thermal injury (burns, frostbite) (Gelfand et al., 1982, J. Clin. Invest. 70:1170; Demling et al., 1989, Surgery 106:52-9), hemodialysis (Deppisch et al., 1990, Kidney Inst. 37:696-706; Kojima et al., 1989, Nippon Jenzo Gakkai Shi 31:91-7), and post pump syndrome in cardiopulmonary bypass (Chenoweth et al., 1981, Complement Inflamm. 3:152-165; Chenoweth et al., 1986, Complement 3:152-165; Salama et al., 1988, N. Engl. J. Med. 318:408-14). Both complement and leukocytes are reported to be implicated in the pathogenesis of adult respiratory distress syndrome (Zilow et al., 1990, Clin. Exp. Immunol. 79:151-57; Langlois et al., 1989, Heart Lung 18:71-84). Activation of the complement system is suggested to be involved in the development of fatal complication in sepsis (Hack et al., 1989, Am. J. Med. 86:20-26) and causes tissue injury in animal models of autoimmune diseases such as immune-complex-induced vasculitis (Cochrane, 1984, Springer Seminar Immunopathol. 7:263), glomerulonephritis (Couser et al., 1985, Kidney Inst. 29:879), hemolytic anemia (Schreiber & Frank, 1972, J. Clin. Invest. 51:575), myasthemis gravis (Lennon et al., 1978, J. Exp. Med. 147:973; Biesecker & Gomez, 1989, J. Immunol. 142:2654), type II collagen-induced arthritis (Watson & Townes, 1985, J. Exp. Med. 162:1878), and experimental allergic neuritis (Feasby et al., 1987, Brain Res. 419:97). The complement system is also involved in hyperacute allograft and hyperacute xenograft rejection (Knechtle et al., 1985, J. Heart Transplant 4(5):541; Guttman, 1974, Transplantation 17:383; Adachi et al., 1987, Trans. Proc. 19(1):1145). Complement activation during immunotherapy with recombinant IL-2 appears to cause the severe toxicity and side effects observed from IL-2 treatment (Thijs et al., 1990, J. Immunol. 144:2419).
Complement may also play a role in diseases involving immune complexes. Immune complexes are found in many pathological states including but not limited to autoimmune diseases such as rheumatoid arthritis or SLE, hematologic malignancies such as AIDS (Tayler et al., 1983, Arthritis Rheum. 26:736-44; Inada et al., 1986, AIDS Research 2:235-247) and disorders involving autoantibodies and/or complement activation (Ross et al., 1985, J. Immunol. 135:2005-14). Inada et al. reported that erythrocyte CR1 has a functional role in the removal of circulating immune complexes in autoimmune patients and may thereby inhibit the disposition of immune complexes within body tissue (Inada et al., 1989, Ann. Rheum. Dis 4:287). A decrease in CR1 activity has been associated with clinical disease state in ARC and AIDS patients (Inada et al., 1986, AIDS Res. 2:235).